Containment system for storage systems having discontinuities

ABSTRACT

A system for sealing a liner about an internal member in a structural envelope, such as a liquid storage tank. A mechanical seal is located between the liner&#39;s first edge and the structural envelope and forms secondary containment between the structural envelope and the liner. A plate sealably attaches outside the outer perimeter of the mechanical seal to either the inner or outer surface of the structural envelope and forms a continuous envelope extension from the liner to the discontinuity. The plate provides secondary containment between the continuous envelope extension and the structural envelope. Tertiary containment is achievable through use of a liner boot, the outer edge attached to the liner outside the perimeter of the mechanical seal and the inner edge mechanically sealed to the discontinuity.

FIELD OF THE INVENTION

[0001] The invention involves methodology and apparatus for providingsecondary containment for structural enclosures such as tanks, moreparticularly for sealing around discontinuities in the tank such asinternal structure and penetrations of the structural envelope.

BACKGROUND OF THE INVENTION

[0002] This invention relates to storage tanks that contain a variety offluids and that are lined with impermeable and sealed internalgeosynthetic liners. Such liners are also referred to in the art asstorage tank bladder seals, as set forth in U.S. Pat. No. 5,558,245, orleakage protection liners as set forth in U.S. Pat. No. 6,431,387.

[0003] The tank storage industry has been slow to adopt sealed internalgeosynthetic liners because of the difficulty in sealing the linermembrane to or around discontinuities or internal structures that attachto the structural envelope such as to the tank's containing wallsincluding floors. Discontinuities include support beams, sumps, stands,gauge boards, and penetrations in the internal floor or walls such aspipes, manways, heating coils, or other penetrations.

[0004] The prior art methodology for sealing internal geosyntheticliners about structures attached to the floor, such as tank columns,consists of three main approaches. The first approach is to extend orbring the liner membrane underneath the tank column, however this placesthe membrane on the tank floor at risk. The impact of weight andmovement of these structures introduces significant uncertainty aboutthe ability of the membrane to withstand the forces of the structure,and the membrane may be cut, crushed or otherwise damaged.

[0005] The second approach, if the column is round, is to bring themembrane up to the column, and then to seal a boot (a membrane welded ina cylindrical shape, with a brim attached at one end of the cylinder andextending out from the cylinder), around the column and to the membraneon the floor, and then to seal the throat of the cylinder of the boot tothe column, using a form of mechanical clamping or banding to bind theboot to the column. This approach also poses risks to the functioning ofthe liner. The hand or detail welding of the boot to the membrane on thefloor is prone to leak and difficult to test. In addition, clamping orbanding systems are often not sufficiently resistant to fluidpenetration. This system introduces a weak link in the liner at apenetration which exposes the entire envelope to a breach. As a resultof the high failure rates of tanks employing these two liner methods,sealed internal geosynthetic liners have not been widely adopted exceptin situations where there are few or no tank penetrations or attachmentsbelow the liquid level of the tank.

[0006] A third, less common, but more effective approach is to usemechanical compression seals for use in sealing the liner directly tothe tank floor about or encircling the discontinuity. Mechanicalcompression systems employ bolts or fasteners attached to the floor ofthe tank, with a batten or punched bar, to compress a system of gaskets,bonding or adhesive material and the membrane to the floor of the tank,or to a flange on the floor, around the structure or attachment. Whilethis approach can provide a more reliable compression and a better sealthan the approaches noted above, the secondary containment is limited tothe main envelope and such advantages are lost specifically at thediscontinuity, such as where the structures are attached directly to thefloor. A fundamental requirement of most modern regulatory schemes isthat storage tanks are design to provide dual containment. Regulatorswill often not approve the omission or loss of dual containment thatoccurs in selected areas in the tank when this third approach is used.

[0007] The prior art for sealing internal geosynthetic liners aroundpenetrations through tank walls and through the internal geosyntheticmembrane covering such walls, such as around pipes for incoming oroutgoing fluid, consists of several methods. A first method is appliedin the case of tanks having penetrations at the point where pipes attachto the tank. Pipes are fitted with a flange that attaches to a flange onthe tank. The geosynthetic membrane is sandwiched between the flangeswith the conventional gasket for sealing therewith around the flange.There are several limitations with this approach. The penetration of thepipe at the flange itself is not covered. Any movement of the pipeattached to the flange exerts considerable leverage on the gasket andmembrane, deforming the membrane and breaking the seal, andcontaminating the interstitial containment space. In addition, leakingcan occur behind the flange, again contaminating the interstitial space.

[0008] A second method, more often used with tanks having welded inplace penetrations, is to fabricate a boot that seals to the membrane onthe inner wall, and then to seal the constricted throat of the bootaround the pipe penetration using a form of clamps or bands to bind thegeosynthetic membrane to the pipe. This approach has limitations similarto liner boot seals about columns as discussed above. Hand or detailseams for boots are less reliable than machine wedge or dielectricseams, and more prone to leakage and failure. Clamping or bandingsystems are not necessarily sufficiently resistant to fluid penetration.

[0009] A problem unique to pipe penetrations is that a boot system forpipe penetrations on the tank wall does not protect the interstitialspace (the isolated area between the tank and the geosynthetic membrane)from leaking that may occur at the junction of the pipe and the tankwall behind the membrane. In the event of such leaks, the entireinterstitial area between the geosynthetic membrane and the inner tanksurface becomes contaminated with fluid. If the interstitial space ismonitored, the monitor will indicate a leak, even though the membranemay not have been the cause of the leak. In addition, because of themovement of incoming pipes, or shifting of the membrane on the innertank surfaces, the boot can be stretched and damaged by these forces,resulting in liner failure. These types of penetration seals cannot beisolated, and are difficult to test. These design problems contribute tothe perception of unreliability of sealed internal geosynthetic liners.

[0010] Finally, a third method to deal with penetrations is to adherethe geosynthetic membrane to the wall of the tank around thepenetration, using both an adhesive material, and compressing thematerial to the wall, using bolts or fasteners attached to the wall ofthe tank, with a batten or punched bar to compress a system of gaskets,bonding or adhesive material and the membrane to the wall of the tank,around the pipe penetration. While this method provides a stronger andmore reliable seal, it suffers from the same limitations of this sameapproach when used on a tank floor. The dual containment provided by thefirst two approaches is lost in this third approach where the point ofpenetration, which is the most susceptible to leaks, loses dualcontainment, and may not be acceptable where regulatory or environmentalrequirements require dual containment. In addition, the metal in thisunprotected area of the tank and penetration is not protected fromcorrosion from the fluids in the tank.

[0011] There is a need for a system which enables an internalgeosynthetic liner to seal effectively and reliably around attachments,structures and penetrations while providing secondary or tertiarycontainment including at such discontinuities.

SUMMARY OF THE INVENTION

[0012] The objects of the invention are achieved through a system thatprovides for sealing an internal geosynthetic liner around or overdiscontinuities in the structural envelope so as to form dual ortertiary containment at the internal members, attachments or otherdiscontinuities including columns, manways, pipes, stands, and gauges. Amore reliable and stronger seal is the result.

[0013] Accordingly, in one broad aspect of the invention, a system sealsa liner about a discontinuity at a structural envelope for containingfluid. The liner forms secondary containment between the liner and aninner surface of the structural envelope. The system comprises: anopening in the liner for forming a first edge about the discontinuity; amechanical seal circumscribed at an outer perimeter about thediscontinuity and located between the liner's first edge and thestructural envelope for forming secondary containment between thestructural envelope and the liner; and a secondary seal attached at aperimeter attachment to one of the inner or an outer surface of thestructural envelope at the discontinuity, the perimeter attachment beingoutside the outer perimeter of the mechanical seal for forming acontinuous envelope extension from the liner to the discontinuity andfor forming secondary containment between the continuous envelopeextension and the structural envelope.

[0014] In another aspect, the system comprises a plate sealably attachedat its perimeter attachment to the inner surface of the structuralenvelope. The plate spaces the discontinuity from the structuralenvelope and is sealably attached to the structural envelope by adiscontinuous weld at the plate's perimeter attachment. The mechanicalseal seals the plate to the liner.

[0015] In yet another aspect, the system comprises a discontinuitypenetrating the structural envelope at a first penetration and attachedto the structural envelope at the first penetration. The plate issealably attached at its perimeter attachment to the outer surface ofthe structural envelope. The discontinuity penetrates the plate at asecond penetration and is sealably attached at the second penetration.The mechanical seal seals the liner to the inner surface of thestructural envelope.

[0016] In a final aspect of the invention the system comprises atertiary seal. The tertiary seal is a liner boot with an outer edgeattached to the liner outside the perimeter of the mechanical seal andextends inward to an inner edge. A second mechanical seal between theliner boot's inner edge and the discontinuity forms secondarycontainment between the continuous envelope extension and the structuralenvelope.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a sectional view of a typical prior art boot welded to aliner and banded to a penetration through a barrier such as a wall or afloor;

[0018]FIG. 2 is a sectional view of a tank lined with a prior artmethodology showing a sealing extending under the base of a centralcolumn;

[0019]FIG. 3 is a sectional view of a tank lined with a prior artmethodology having a boot welded to the liner and extendingsignificantly up the column;

[0020]FIG. 4 is a sectional view of a prior art compression sealpracticed by the applicants for sealing about a penetration through astructural envelope;

[0021]FIG. 5a is a cross sectional view of a sealing system according toone embodiment of the invention, showing a mechanical seal to a plateunder a column the plate being supported on a structural envelope orbarrier such as a tank floor and forming a continuous envelope extensionwith the liner;

[0022]FIG. 5b is a close-up cross-sectional view of the mechanical sealat the periphery of the plate of FIG. 5a;

[0023]FIG. 6a is a cross sectional view of a sealing system according toanother embodiment of the invention, showing a mechanical seal to aplate under a column and a boot seal about the mechanical seal, theplate being supported on a structural envelope or barrier such as a tankfloor;

[0024]FIG. 6b is a close-up cross-sectional view of the mechanical sealat the periphery of the plate of FIG. 6a;

[0025]FIG. 7a is a cross sectional view of a sealing system according toanother embodiment of the invention, showing a mechanical seal to thestructural envelope with an exterior plate, or secondary plate on theexterior surface of the structure, opposite and surrounding themechanical seal;

[0026]FIG. 7b is a cross sectional view of a sealing system according toanother embodiment of the invention, showing a mechanical seal to thestructural envelope adjacent a penetration and a boot seal about themechanical seal;

[0027]FIG. 8 is a cross section of a plastic boot;

[0028]FIG. 9 is a plan view of sectional compression bars applied tomechanical seals around any discontinuity; and

[0029]FIG. 10 is a cross sectional view of a sealing system according toanother embodiment of the invention, showing a mechanical seal to thestructural envelope adjacent a penetration, secondary containmentprovided by a plate sealably attached to the exterior of the structuralenvelope, and a boot seal about the mechanical seal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030] In the prior art, attempts have been made to seal structuralenclosures or envelopes with a lining. These envelopes are typicallystorage tanks made of steel and are configured to support hydrostaticloading of liquids contained therein. Steel envelopes are subject tocorrosion and all envelopes including those of plastic are prone toleaks. Such enclosures are fit with a liner when possible.

[0031] With reference to FIG. 1, a prior art system illustrates astructural envelope 10, or storage tank 12, which is sealed to adiscontinuity 16 using a liner 18 extended to fit to the discontinuitywith a boot 20. FIG. 2 illustrates a further prior art situation wherethe liner 18 runs directly under the discontinuity 16 resting on thefloor of the storage tank 12. FIG. 3 illustrates yet another alternateprior art approach, illustrating a very tall boot 20 extending upwards,preferably above an expected liquid level. None of these approaches havegarnered widespread acceptance.

[0032] With reference to FIG. 4, one prior art approach to improve thesealing and security of the liner 18 to the structural envelope 10,practiced by the Applicants, includes bringing the liner 18 adjacent tothe discontinuity 16, and mechanically sealing the liner 18 with meanssuch as a compression device 30. As shown in FIG. 5b, a compression bar32 is placed over a threaded stud 34 affixed to a plate 60. Gaskets 36sandwich the liner 18 while a washer 38 and a nut 40 are tightened ontothe threaded stud 34 to compress and completely seal the liner 18.However, referring back to FIG. 4, Applicants recognize that nosecondary containment results at the discontinuity 16. Thus, thediscontinuity 16 is subjected directly to the internal environment andany leak that develops would be directed to the outside environment.

[0033] Further, the structural envelope 10 of fluid-containing wallsincluding floor surfaces of storage tanks 12 generally require sealsaround discontinuities 16 in the structural envelope 10. Discontinuities16 include structures that penetrate through a surface 42 of thestructural envelope 10, as well those that attach to the surface 42.

[0034] The principles and design of Applicants' solution are applicablewhether addressing either type of discontinuity 16 identified above. Inpractice, most penetrating discontinuities 16 occur through the wall ofthe tank 12, and involve the piping of fluids in and out of the tank 12,or manways. Other discontinuities 16 involve such things as columns,gage boards, or pipe stands attached to the structural envelope 10.

[0035] Turning then to one embodiment of the present invention in FIGS.5a, 5 b, a system for sealing the liner 18 about the discontinuity 16 isshown. A discontinuity 16, such as an internal member 44, bears againstor is attached to the surface 42 of the storage tank 12. As shown inFIG. 5a, an opening formed in the liner 18 of geosynthetic membraneallows the liner 18 to cover an inner surface of the tank 46, andextending up to a first edge 50 circumscribed about the discontinuity16. The compression device 30 seals the liner 18 to the structuralenvelope 10. Accordingly, secondary containment is provided in aninterstitial space 56 formed between the liner 18 and the structuralenvelope 10 and terminating at the compression device 30 and first edgeof the liner 50.

[0036] As seen in FIG. 5b, interstitial material 57 is placed preferablybetween the liner 18 and the structural envelope 10. The interstitialmaterial 57 is fluid-transmissive and is typically applied to enablefluid flow between the liner 18 and the structural envelope 10. Examplesof suitable interstitial materials 57 are geotextiles and geogrids,commonly used in construction and landscaping applications.

[0037] Herein, geosynthetic membranes (geomembranes) are understoodgenerally to be impermeable sheets composed of synthetic materialsincluding PVC, fluorothermoplastic, urethane, polypropylene, orhigh-density polyethylene (HDPE). Geomembranes are commonly used aslandfill liners, pond liners and in capping applications. Geotextilesare porous, synthetic fabrics, made of woven or non-woven fibers.Geogrids are molded rigid, porous matts made from high-densitypolyethylene (HDPE) or other synthetic resins. Geogrids are generallymore resistant to pressure than geotextiles, but more difficult to workwith, and more expensive.

[0038] For completing secondary containment including at thediscontinuity 16, the plate 60 is provided which separates thediscontinuity 16 from the structural envelope 10. The discontinuity 16is initially separated from the surface 42, and the plate 60, typicallyof a material complementary with the structural envelope 10, is insertedtherebetween. The plate 60 has a support area larger than the area ofthe contact point of the internal member 44 (preferably providing aperimeter of about 12 inches around the contact point) and extends to aperimeter outside the perimeter of the mechanical seal. The plate 60 issecured to the structural envelope 10 such as through welding (metal orplastics) or is otherwise attached to the surface 42, preferably withoutfurther penetration of the surface 42. An interstitial area 58 is formedbetween the plate 60 and the structural envelope 10. The composition ofthe material of the plate 60 may depend on whether the plate 60 willcome in contact with the fluid of the tank 12, and the corrosive natureof such fluids.

[0039] The plate 60 is attached to the inner surface 46 using a weldingpattern that is continuous or discontinuous. If the welding pattern isdiscontinuous the interstitial area 58 is in communication with theliner's interstitial space 56. If a plate is optionally attached outsidethe inner surface 46 then the welding pattern would be continuous.

[0040] The addition of the plate 60 provides a number of advantages. Theplate 60 allows the attachment of almost any kind of internal member 44thereto and compensates for seams and other imperfections on the surface42, providing a smooth clean surface 42 for a weld 62 and providessufficient workman's assess and a surface 42 for the attachment of theliner 18 of geosynthetic membrane. The use of a stitch weld, or anyother attachment process, that allows fluid to flow from theinterstitial area 58 under the plate 60 to the liner's interstitialspace 56 accomplishes secondary containment under the internal member44. A preferred configuration of the plate 60 is to maintain arectangular plate with rounded corners, allowing for the use of standardsized rectangular, mechanical compression pieces and corners, with sidesof varying lengths.

[0041] Best seen in FIG. 5b and FIG. 9, the preferred compression device30 is a series of threaded studs 34, preferably with 2 inch spacing fromcentre to centre, welded to and extending from the plate 60. An openingis cut in the liner 18 so the liner is formed around the discontinuity16 to form the first edge 50 which is punched with a plurality of holescorresponding and aligning with the spacing of the studs 34. Thecompression bar 32 is also punched in a pattern that matches the seriesof threaded studs 34 welded to the top of the plate 60. The bar 32 isplaced over the threaded studs 34 and gaskets 36. The washer 38 and nut40 are tightened to compress the device 30 onto the liner 18 creating amechanical seal 54.

[0042] Accordingly, secondary containment is also provided by theinterstitial area 58 at the discontinuity 16 formed between the plate 60and the structural envelope 10.

[0043] Typically, the mechanical seal 54 further comprises flexiblegaskets 36. The gasket material is a compressible material punched withcorresponding stud holes that creates constant and uniform pressure whencompressed. It is preferable that the gasket material have a memory tomaintain constant pressure over time. While it may in some instances bepreferable to place only the gaskets 36 between the liner 18 and theplate 60 or structural envelope 10, adhesive and sealant 64 arepreferably also applied to both sides of the gaskets 36. As gaskets 36also come in contact with the fluids, it is recommended they be selectedto provide chemical resistance to the fluids.

[0044] Adhesive or sealant 64 can be applied between the liner 18 andthe plate 60 to aid in sealing small channels that may exist wheremembrane seams overlap, or deal with surface imperfections. The sealant64 can take the form of applied liquids or gels, tapes, or mastics.Materials with adhesive qualities have the added advantage of providinga sealing grip between the plate 60 and the liner 18 in the event of thefailure or weakness in the mechanical seal 54. As adhesive or sealant 64will come in contact with tank fluids, it is recommended they beselected to provide chemical resistance to the fluids.

[0045] The mechanical seal 54 provides a reliable seal of the liner 18about internal members 44, structures, attachments, and otherdiscontinuities 16 generally of all shapes and sizes. Further, theinterstitial area 58 under the plate 60 becomes a continuation of thesealed area or interstitial space 56 that exists under the liner 18, andthe plate 60 serves as a sealed continuation of the liner surface, orcontinuous envelope extension, which passes under the discontinuity 16,and thereby provides secondary containment.

[0046] With reference to FIGS. 6a and 6 b, in another embodiment, a boot20 (as seen in isolation in FIG. 8) provides third layer of containment,or tertiary seal, and provides a redundant seal that must be challengedbefore the mechanical seal 54 and plate 60 come in contact with thefluids leaking from the tank 12. The additional seal of the boot 20 alsoprovides protection to the various metal components of the system, suchas the plate 60, the studs 34, washers 38, nuts 40 and the compressionbar 32.

[0047] Accordingly, in this embodiment, this alternate design adds theflexible geosynthetic boot 20 to cover the mechanical seal 54, the baseof which has an outer edge 66 which extends beyond the perimeter of themechanical seal 54 and is fused or adhered directly to the liner 18. Aninner edge 68 is sealed to the discontinuity 16 using a secondmechanical seal for creating an additional barrier to liquid entry atthe discontinuity 16. Further, mechanical stress on the liner 18 such asthat imposed by shifting or pulling is now absorbed by the robustmechanical seal 54 and thereby relieves stress on the more fragile boot20 which has points of inherent weakness including the welding at seams.

[0048] This boot 20 becomes an additional continuous envelope extension,extending to the liner 18 and can be prefabricated such as that formedfrom two planes of liner 18 material. One plane involves a large flatmembrane that is circular or rectangular with rounded corners, and witha circular hole in the centre of the liner 18, the outer perimeterforming the boot's outer edge 66. The second plane is a rectangularmembrane fused or adhered into a cylindrical shape, the upper extremityforming the boot's inner edge 68. One end of the cylinder membrane isfused or adhered to the circular hole in the first flat membranecreating a single boot 20, with a throat 70 through which thediscontinuity 16 can pass.

[0049] The upper extremity or throat 70 of the boot 20 is then sealed tothe discontinuity 16 with a single or multiple circular bands of steel72, or other banding material which provide a further barrier to liquidentry where the geosynthetic material throat 70 meets the discontinuity16. A preferred configuration of banding consists of three stainlessbands around the discontinuity 70. Adhesive or sealant 64 can also beadded to provide the necessary seal to the boot 12 and the discontinuity16.

[0050] If the cross-section of a discontinuity 16 is irregular, otherfastening techniques, particularly glues or adhesives 64, can be used toconnect the throat 70 of the boot 20 and seal therebetween. Thisadditional boot 20 provides a barrier through which liquid must passbefore it can come in contact with the mechanical seal 54.

[0051] The fabrication techniques of the boot 20 include the use ofgeosynthetic adhesives, or hand welding equipment, ensuring a continuousliner 18 without any channels or holes.

[0052] With reference to FIG. 7a, another embodiment of the inventionaddresses discontinuities 16 such as those which penetrate through thesurface 42 of storage tanks 12 where the geosynthetic liner 18 coversthe inner surface of the tank 46.

[0053] In one aspect, the lack of secondary containment in the case ofthe prior art illustrated in FIG. 4 is overcome through the addition ofmeans for secondary containment on an outer surface of the tank 74.Accordingly, the liner 18 is sealed internal to the envelope 10 and upto the penetrating discontinuity 16, and a containment means such as aplate 60 sealably connected to the outer surface of the tank 74, formssecondary containment.

[0054] The mechanical seal 54 is used to obtain secondary containment atthe penetration point by the installation of the plate 60 installed onthe outer surface of the tank 74 and about the discontinuity 16. Theplate 60 has an opening in its interior for the discontinuity 16 to passthrough. If there were no penetration, the plate 60 would be continuous.The plate 60 is sealed around its outer periphery so as to provide asealed area extending beyond the perimeter of the mechanical seal 54 onthe inner surface of the tank 46, on the outer surface of the tank 74.The plate 60 is sealed around its inner periphery to the discontinuity16. The combination of secondary containment on the outer surface of thetank 74 in the penetration region, with the secondary containment on theinner surface of the tank 46 around the penetration region, provides theliner 18 with secondary containment that includes the penetratingdiscontinuity 16.

[0055] There exists an opportunity to monitor isolated interstitialregions separately for leaks using a monitoring device 80, giving riseto variations of the invention, where a mechanical seal 54 results inthe establishment of interstitial regions that are isolated from otherinterstitial regions. FIG. 10 shows the simple hole 82 and plug 84 thatcan be opened to monitor for leaks in interstitial area 58, between theouter surface of the tank wall 74 and the plate 60. Interstitial spacessealed and isolated by boot systems around wall penetrations can bemonitored directly through simple valve openings through the tanksurface 42. Any fluid leak into the interstitial area 58 can be detectedby opening the valve. When the tank 12 is in service the pressuresagainst the boot 20 will provide a flow of fluid through the tanksurface 42.

[0056] With reference to FIG. 7b, another embodiment of the inventionaddresses the discontinuity 16 that pass through the wall of storagetanks 12 where the geosynthetic liner 18 covers the inner surface of thetank 46. While the design is similar, the fact that the pipe penetratesthe wall means that secondary containment is now completed by the use ofthe boot 20 embodiment that covers the penetration, as opposed to theplate 60 according to FIG. 7a.

[0057] With reference to FIG. 10 another embodiment is a hybrid of FIG.6a and FIG. 7b, the plate 60 may also be used as shown in FIG. 6a, yetthe discontinuity 16 sealably penetrates the plate 60. The boot 20 isthen applied as in FIG. 7b to provide tertiary containment. The boot 20added to the mechanical seal 54 is particularly advantageous over priorart systems including those shown in FIGS. 1-4.

[0058] The design of the boot 20 is the same as FIG. 6a, 6 b. The designof the mechanical seal 54 is the same as that in FIG. 6a except that thethreaded studs 34 are welded onto the inner surface of the tank 46, asopposed to the plate 60. The flexible geosynthetic boot 20 has a basewhich extends beyond the perimeter of the mechanical seal 54 on thetank's inner surface 46, and is described in detail in respect of themechanical seal 54 above. This boot 20 is fused or adhered directly tothe liner 18 on the tank's 12 penetrated surface 42. The throat 70 ofthe boot 20 is then sealed to the discontinuity 16 with single ormultiple circular bands of steel 74, or other material, and/or withchemical adhesive 64 or bonding, providing a further barrier to liquidentry where the geosynthetic throat 70 meets the discontinuity 16.

[0059] Like the mechanical seal 54 for discontinuities 16 shown in FIG.6a, 6 b, this also creates a distinct and isolated interstitial area 58around the penetration point, an area which is at higher risk for leaks.This distinct and isolated interstitial area 58 can be monitored and isuseful for detecting a leak around the joint before the maininterstitial space 56 is contaminated, meeting secondary containmentrequirements at the joint. The boot 20 covers, and protects the metalmechanical seal 54 from corrosion from tank fluids, and provides anadditional or second barrier to liquid entry. An alternative embodimentadds to the foregoing a hole 82 with a removable plug 84 in the exteriorplate 60, to allow for the monitoring of the mechanical seal 54 aroundthe discontinuity 16.

[0060] Thus, it is apparent that there has been provided, in accordancewith the invention, a system for sealing a structural envelope 10 thatfully satisfies the objects, aims and advantages set forth above. Whilethe invention has been described in conjunction with specificembodiments such as a storage tank 12, it is evident that it is equallyapplicable to any discontinuity 16 of an envelope 10 including manyalternatives, modifications and variations which will be apparent tothose skilled in the art and in light of the foregoing description.Accordingly, it is intended to embrace all such alternatives,modifications and variations as fall within the spirit of the appendedclaims.

The embodiments for which an exclusive property or privilege is claimedare defined as follows:
 1. A system for sealing a liner about adiscontinuity at a structural envelope for containing fluid, the linerforming secondary containment between the liner and an inner surface ofthe structural envelope, the system comprising: an opening in the linerfor forming a first edge about the discontinuity; a mechanical sealcircumscribed at an outer perimeter about the discontinuity and locatedbetween the liner's first edge and the structural envelope for formingsecondary containment between the structural envelope and the liner; anda secondary seal attached at a perimeter attachment to one of the inneror an outer surface of the structural envelope at the discontinuity, theperimeter attachment being outside the outer perimeter of the mechanicalseal for forming a continuous envelope extension from the liner to thediscontinuity and for forming secondary containment between thecontinuous envelope extension and the structural envelope.
 2. The systemof claim 1 wherein the secondary seal is a plate.
 3. The system of claim2 wherein the plate is sealably attached at its perimeter attachment tothe inner surface of the structural envelope for spacing thediscontinuity from the structural envelope, the mechanical seal beingbetween the plate and the liner.
 4. The system of claim 3 wherein theplate and the structural envelope are sealably attached by adiscontinuous weld at the plate's perimeter attachment.
 5. The system ofclaim 3 wherein the plate and the structural envelope are sealablyattached by a continuous weld at the plate's perimeter attachment. 6.The system of claim 2 wherein the plate and the structural envelope aresealably attached by a continuous weld at the plate's perimeterattachment.
 7. The system of claim 2 wherein the discontinuitypenetrates the structural envelope at a first penetration and isattached to the structural envelope at the first penetration, andwherein: the plate is sealably attached at its perimeter attachment tothe outer surface of the structural envelope; the discontinuitypenetrates the plate at a second penetration and is sealably attached atthe second penetration; and the mechanical seal being between the linerand the inner surface of the structural envelope.
 8. The system of claim7 wherein the structural envelope is a wall of a tank.
 9. The system ofclaim 2 wherein a monitoring device is added to between the structuralenvelope and the plate for detection of a breach of the secondarycontainment.
 10. The system of claim 1 wherein the structural envelopeis a floor of a tank.
 11. The system of claim 5 wherein the structuralenvelope is a floor of a tank.
 12. The system of claim 1 furthercomprising: a tertiary seal having a liner boot with an outer edgeattached to the liner outside the perimeter of the mechanical seal, theliner boot extending inward to an inner edge; and a second mechanicalseal between the liner boot's inner edge and the discontinuity forforming tertiary containment between the continuous envelope extensionand the structural envelope.
 13. The system of claim 5 furthercomprising: a tertiary seal having a liner boot with an outer edgeattached to the liner outside the perimeter of the mechanical seal, theliner boot extending inward to an inner edge; and a second mechanicalseal between the liner boot's inner edge and the discontinuity forforming tertiary containment between the continuous envelope extensionand the structural envelope.
 14. Apparatus for sealing a liner about adiscontinuity at a structural envelope for containing fluid, the linerforming secondary containment between the liner and an inner surface ofthe structural envelope, the system comprising: a mechanical sealcircumscribed at an outer perimeter about the discontinuity and locatedan opening in the liner for forming a first edge about thediscontinuity, the liner's first edge and the structural envelope forforming secondary containment between the structural envelope and theliner; a plate attached at a perimeter attachment to one of the inner oran outer surface of the structural envelope at the discontinuity, theperimeter attachment being outside the outer perimeter of the mechanicalseal for forming a continuous envelope extension from the liner to thediscontinuity and for forming secondary containment between thecontinuous envelope extension and the structural envelope.